This invention relates to wastewater treatment, and in particular to biological oxidation treatment in at least two stages comprising: decomposing at least a portion of organic pollutants in the stages by at least partially gas-treating, e.g., aeration, in the presence of activated sludge, discharging the so-treated wastewater-activated sludge mixture from the treatment stages, and dividing in a post clarification stage into purified water and sludge. The sludge divided in the post clarification stage is at least partially recycled into the treatment stages. This invention also relates to an apparatus for conducting the process.
In such a process, wastewater is conventionally initially mixed, in the first stage, with activated sludge with the simultaneous introduction of molecular oxygen in the form of air or a gas having a higher oxygen concentration than air. By virtue of the activity of the aerobic microorganisms contained in the activated sludge, the organic pollutants of the wastewater are, decomposed or degraded, the specific mechanism being conversion, in part, into bacterial substance and, in part, into CO.sub.2 and water, the overall effect in the first stage being extensive oxidation of the carbon compounds. The microorganisms in this stage require oxygen to maintain their metabolic function, as well as for growth and multiplication, for rapidly degrading the organic substances in the wastewater. The microorganisms are relatively compact, i.e., sufficiently low density and low surface to volume ratio, so that they can be removed as settled sludge in a post clarification tank. The purified wastewater and sludge are then discharged from the post clarification tank. A portion of the discharged sludge, containing settled microorganisms, is recycled into the first treatment stage to maintain a desired quantity of microorganisms in the wastewater-activated sludge mixture, whereby the biological process operates continuously.
In addition to the biological degradation of the organic substance, a chemical degradation is simultaneously conducted in a second stage comprising oxidizing ammonium nitrogen to nitrite and nitrate in the presence of primarily autotrophic bacteria. The bacteria in this second stage is supplied with air or a gas having a larger vol-% of oxygen than air.
The chemical degradation conducted in this second stage is the so-called nitrification of the wastewater, and is conducted in most cases after the major portion of biological degradation has occurred in the first stage. Heretofore an intermediate clarification tank was provided between the first and second stages. This intermediate clarification, however, has proven to be uneconomical. Nonetheless, although expensive, the intermediate clarification was required because if omitted, it was difficult to maintain the very slowly growing nitrifying bacteria at the level necessary to conduct nitrification. More specifically, since both biological degradation and nitrification occur simultaneously, sludge is produced and the nitrifying bacteria would in the absence of an intermediate clarification be discharged as excess sludge, together with the very rapidly growing organic compound-degrading bacteria. As a result of this constant removal of nitrifying bacteria, the ammonium nitrogen present can no longer be reduced to the required values. The intermediate clarification, therefore, was considered necessary so as to enable nitrification to be conducted both in the presence of nitrifying bacteria and in the substantial absence of organic compounds and their associated bacteria.